The invention is directed to a thin carbon coating on an optical waveguide fiber. The coating acts to improve the waveguide fiber performance. More particularly, a thin carbon coating, formed on the clad glass layer of the waveguide fiber, has been found to improve dynamic fatigue performance of the waveguide fiber. In addition, the carbon coating markedly improves resistance to delamination between the polymeric coating and the waveguide fiber, under severe environmental conditions such as immersion in water.
The concept of coating optical waveguide fibers is known in the art. Polymer coatings have been developed to protect the waveguide fiber from handling damage as well as to reduce the impact of bending on waveguide attenuation. Also, hermetic coatings have been developed to seal the waveguide fiber from OHxe2x80x94 ions, which enable growth of waveguide surface flaws when the waveguide is under stress. A hermetic coating also is important in protecting the waveguide from corrosive materials, and gasses, particularly hydrogen, which can diffuse into the waveguide and cause increases in attenuation.
Of the several types of coating material tested in the search for a hermetic coating, carbon has been found to be most compatible with the manufacture, packaging and use of a waveguide fiber.
The thickness of the carbon layer sufficient to provide hermeticity has been found to be in the range of 1000xc2x0 A or greater. In U.S. Pat. No. 4,964,694, Oohashi et al., carbon coating thickness of the range of 1000 to 6000xc2x0 A is taught ( col. 3, II. 29-34). Thickness less than 1000xc2x0 A tend to allow pinhole formation in the coating. Thickness greater than 6000xc2x0 A tend to crack and peel from the waveguide surface. Hermeticity is also measured in terms of resistance to the passage of hydrogen through the coating. See, for example, U.S. Pat. No. 5,000,541, DeMarcello et al., col. 4, II. 19-39. At col. 5, II. 11-15, of ""541 DeMarcello, a carbon layer of thickness of 1000xc2x0 A is noted as providing a barrier to the diffusion of hydrogen.
The manufacturing and cost penalties which arise from the incorporation of a carbon coating step into the waveguide fiber manufacturing process are:
drawing speed is limited by the requirements of carbon coating thickness and integrity;
an additional on line measurement of carbon coating thickness must be added to the draw feedback control loop;
additional quality control testing for hermeticity must be done; and,
the black color of the waveguide complicates the process of coloring the polymer layer to color code multiple fiber assemblies.
The invention overcomes the drawbacks of achieving hermeticity while maintaining some of the benefits thereof. Additional unexpected benefits also derive from the presence of the thin carbon coating.
Thus, a first aspect of the invention is an optical waveguide fiber coated with a carbon layer having a thickness no greater than about 100xc2x0 A. It is contemplated that thickness no greater than 50xc2x0 A are sufficient. As carbon coating becomes thinner, one may expect the waveguide properties to approach those of a non-carbon coated waveguide fiber. Some benefit in terms of carbon coated waveguide fiber performance may be expected at thickness about 10 xcexcm. The thin carbon layer is distinguished from a hermetic carbon coating by its permeability to fluids, such as hydrogen. However, the dynamic fatigue constant, which is about 20 for a silica clad waveguide, is greater than about 25 in the case of a waveguide having a thin carbon layer. This increase is quite significant in light of the fact that the fatigue constant appears as an exponent in the equation predictive of time to failure.
In addition to the characterization of the thin carbon layer by its thickness, the layer may also be characterized by its resistance per unit length, which is no greater than about 4 Mega-ohms/cm (Mxcexa9/cm). The thin layer of carbon is bonded to the waveguide clad glass layer. The layer is colored a light gray.
A second aspect of the invention is the surprising discovery that the thin carbon layer acts to essentially prevent delamination of the polymer coating. The integrity of the waveguide fiber having a protective polymer coating is such that substantially no attenuation increase was induced by immersing the carbon and polymer coated waveguide in water for extended time periods. The standard environmental tests call for room temperature water soak and hot water soak, about 65xc2x0 C., for 30 days. The tests on the novel carbon coated waveguide fiber were extended to 128 days, in both room temperature and hot water, and still substantially no induced attenuation was observed.
An additional benefit of the coating results from its light gray color which allows, in contrast to the black hermetic coating, the waveguide fiber to be color coded using methods and pigments known in the art. The colors successfully applied and tested were yellow, white, red, and green. These colors are believed to be the most difficult to apply and the most likely to change in environmental testing.